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EXCURSIOK TO GROVE PARK AND . SATGRDAY, MARCH 20TH, I920. REpORT BY L. DGDLEY STA)fP, M.Sc., F.G.S. Director of the Excursion. A PARTY of about 50 assembled at Grove Park Station at 2.20 p.m., and walked in a southerly direction towards Sundridge Park. On a spur of high ground overlooking the Golf Links, a short halt was made and some features of the local geology explained. The party was here standing on London Clay, and looking eastwards, the high ground occupied by Elmstead Wood (also consisting of London Clay) was seen. The intervening valley, in which the little Quaggy flows, is cut in Oldhaven Sands and Woolwich Beds. The Director remarked that the Eocene beds of South-east London were bv no means horizontal There are two sets of broad gentle folds, approximately at right anglcs*; the one set running North-east to South-west, the other north-west to South-east. An example of the first set is seen in the syncline running from Shooter's Hill to Crystal Palace (London Clay). Shooter's Hill was hidden in the haze, but the high ground of to the South (formed by Blackheath Beds on the south-eastern limb of the syncline) was seen. An anticlinal member of the second series was chiefly responsible for the Chalk inlier of Elmstcad (seen later in the afternoon), and for the North-western prolongation of the Chalk inlier at New Cross. These two, the Shooter's Hill Syncline and the Elmstead-New Cross Anticline practically neutralise each other between and Grove Park, and ncar where the party was standing. Owing also to th e existence of these two sets of folds the Elmstead inlier partakes of the nature of an elongated dome rather than that of a simple anticline. Turning to the left the party followed the lane towards Mot­ ingham across the Golf Links and reached the railway cutting near the north-western end of Elmstead Tunnel. Here the pebbly Basement-Bed of the London Clay, resting on about IO feet of clean Oldhaven Sand.] could be made out although the sides of the cutting were well grassed. Rabbits cause con­ siderable damage by burrowing in the soft sand just below the clayey Basement-Bed. The bottom of the cutting touches Woolwich Shell Beds, which here consist of a brownish clay, full of fluviatile shells-Paludina and Unio-together with estuarine Cyrena: in the lower part. The Director passed round a specimen, which had been obtained in a small excavation ·c. E. N. Bromehead, Sum. Prog, Geol. Surv. for 1913 (1914), p. 28. tWo Whitaker, Geology of London, vol, I. p. 236, Mem. Goal. Surv. (1889). EXCURSION TO GROVE PARK A!'\D CHISLEHl·RST. 151 near the Signal Box, showing all three forms. It was remarked that the Paludina Bed is not of the usual calcareous type, but distinctly a clay bed. After noting some shallow trench sections cut in Oldhaven Sand with a few pebbles (ncar the western corner of the Golf Links) the party proceeded over the hill, following the line of Elmstead Tunnel. The surface of the ground was somewhat confused by Blackheath material from tunnel trial holes. From the hill-top the valley occupied by the Chalk inlier was seen, with high ground of Blackhcath Beds on either side, sloping gently away from the axis of the fold. Descending the hill the next halt was made at the famous Sundridge Rock Pit, situated IOO yards north-east of Elmstead Station. Sir Cyril and Lady Harrison, in whose garden the pit is situated, were waiting to receive the Association. The pit is in pebbly Oldhavcn Sands, which are here up­ wards of 40 fect in thickness and rest on a highly eroded surface of Woolwich Beds. In the railway cutting, close by, they were formerly seen to repose on about 2 or 3 feet of the Bottom Bcd of the Woolwich Series." The false dip of the Blackheath Beds is between IO and 25 degrees to the north, being highest in the upper part.] The upper part consists of banks of oysters iOstrea beUovacensis) cemented into hard rock by calcareous cement (hence the name--Sundridge Rock Pit). The beds arc fossili­ ferous throughout, but the fossils in the loose sands are extremely fragile. The Director remarked on the very definite and dis­ tinct nature of the Blackheath fauna, consisting, as it does, of a mixture of estuarine and marine species. The occurrence of a few estuarine or fluviatile shells in a marine deposit is of comparative little importance as they may have been brought in by emptying into the sea; but the occurrence of marine shells mixed with estuarine in a deposit succeeding one with estua­ rine species only, indicates a marked change of conditions. The Woolwich Shell Beds have essentially a brackish water fauna or in some localities (as at Elmstead) a freshwater one. The succeeding Blackheath Beds have a number of the Woolwich estuarine species together with a considerable proportion of marine species. Although it has been suggested] that they were formed as shingle banks holding back freshwater lagoons, in which the Woolwich Paludina Beds were deposited, i.e., con­ temporaneously, the character of the beds is entirely inconsistent with this idea. The perfect rounding of the pebbles andthe absence of bleaching, point to a distinctly submarine, rather than coastal, origin.§ Moreover, the Blackheath Beds always rest on an

"Ibid. p, 227. tIbid. p.227. :C. E. N. Bromchead, Q,J.G.S•• vol.Jxxv (1919), p. 10. IW. Whitaker, Q.f.G.S., vel, xxii (1866), p.415. Geol. London Basin, Mem. Geol. Sur •• ' vol IV (1872), p. 240. Geol, of London, vol. i (1889), p. 215. Mem, Geol; Sun'. L.D. STAMP,

eroded surface of the underlying Woolwich Beds and even in­ clude rolled fragments of them.* Whilst the assemblage of heavy minerals of the former is identical with that of the under­ lying beds; the percentage (in patches) is very high, as if the sand had been naturally "panned" by current action.t A sample of sand from this pit gave. 208% ; another of Blackheath sand from Eltham Park Railway-cutting gave .325 %; whilst a sample of Thanet Sand from the Willett's pit (visit ed later in the afternoon), only gave .095% and that is considerably above the average for Thanet or Woolwich Beds. The usually " clean" nature of the Blackheath Sands also seems to indicate natural washing of the sand by currents. The fossils found in this pit during the excursion included the following hrackish­ water species :-Cyrcna (Corbicula) cuneiformis Fer., Cyrena spp., ,'vIclania inquinata Defr., Potamides [unatus (Mant) and. Ostrea bellovacensis Lamk.: and the following marine species, Pedunculus (Axina1a) plumstcadiensis Sow., Calypireea trochi­ form is Lamk., Serpula, ?Arca and several examples of Polyzoa (?Flustra) encrusting Ostrea. Small freshwater species (probably floated into the Blackheath Sea) also occur, and several examples of the tiny Planorbis hcmistoma Sow. were found together with small (probably young) Cyrenai, inside the valves of larger Cyrena by Mr. A. Wrigley. Before leaving the pit the President pro­ posed a hearty vote of thanks to Sir Cyril Harrison, who had not only given the Associat ion permission to visit the exposure but had also taken a great deal of trouble in having the pit cleared of all obstacles, so that a truly magnificent section was seen by the visitors. Sir Cyril , replyin g, expressed a cordial welcome to geologists. The party th en proceeded towards Chislehurst, halting at th e disused Chalk-pit at the back of Messrs. W. Willett's premises in Lower Camden (by kind permission of Messrs. Wil­ lett). Th e upper part of the section is in Thanet Sand, with the Bullhead Bed at the hase and resting somewhat irregularly on the Chalk . Th e lower three or four feet of the Thanet Sand is very glauconitic and for six or eight feet from the base is unusu­ ally coherent. It is not unlik e some of the softer varieties of .. tuffeau" occurring in North France at about this horizon. It is a likely rock in which to search for fossils, indeed, some nicely preserved casts of worm burrows had been found , but, unfortunately, the section is very inaccessible. The coherence of th e rock is not due to calcareous matter, but a large amount of argillaceous material (about 25%) was found to be present. It is of interest to note that it was probably from this pit that Miss M. I. Gardiner obtained a sample of sand studied in her pioneer work on the mineralogical constitution of sands.] A -H. A. Baker, Geo;. .\tag., [ 92 0 ~ p. 6: . +Cf. P. G. H. Boswell. Q.j.G.S ., vol. Ixxi ( 1l) 1 .~ ) , p, 566. : " Th e Greensand Bcd allhe Base of th e Th anet Sand." Q.j.G.S. , vol. xliv ([ 888), p. 755. EXCURSION TO GROVE PARK AND CHISLEHURST. I53 curious feature of the Chalk in this locality is the presence of certain indurated layers, one of which, about I8 inches thick, has been utilised to form the roof of artificial" caves" in this. and in neighbouring quarries. After tea at the Hotel, the President moved a hearty vote of thanks to the Director. Some of the members, taking advantage of the remaining daylight of a remarkable spring day, walked up on to Chislehurst Common and saw part of the ex­ tensive plateau, sloping gently with the dip of the beds, to the north. The Common is formed by Blackheath Pebble Beds, and is still largely covered with silver birch, a tree that thrives particularly well on these beds. It was seen that Chislehurst. well deserves its name (A.S. cisil, O. H. Germ, chisil, gravel;. A. S. Hyrst, M. H. Germ, hurst, a wood.) Mr. E. E. S. Brown acted as Excursion Secretary.

REFERENCES.

Maps.-Geological Survey, t in., London District, Sheet 4. Ordnance Survey, r in., Large Sheet Series, Sheet I16. PREVIOUS EXCURSIONS.- 1897. W. WHITAKER and T. V. HOLMES, Proc. Geoi. Assoc.,vol. xv.. p. 108.. 1900. T V. HOLMES. Proc. Geol, Assoc., vol. xvi., p. 522. 1901. 1.'. V. HOLMES., Proc. Geol. Assoc., vol. xvii., p. 136. 1902. T, V. HOLMES and C. \V. OSMAN, Proc, Geol. Assoc., vol. xvii., pp. 368 and 489.